Various relatively small motorized vehicles, such as snowmobiles, all-terrain vehicles (ATV's), tractors, motor scooters, go-carts and golf carts use an endless belt type continuously variable transmission (CTV). Variable transmissions include a variable-input drive/driving/primary pulley or clutch and an output driven/secondary pulley or clutch. The drive pulley is connected to the crankshaft of the engine. The driving pulley is also called the input pulley because it is where the energy from the engine enters the transmission. The second pulley is called the driven pulley because the first pulley is turning it. As an output pulley, the driven pulley transfers energy to the drive shaft of the track drive. Each pulley is composed of a fixed sheave or pulley half that is fixed in the axial direction, and a movable sheave or pulley half, which is movable in the axial direction. A high power metal or rubber belt, such as a V-belt, joins the drive pulley and the driven pulley and rides in the groove between the two sheaves. When the two sheaves of the pulley are far apart, the belt rides lower in the groove, and the radius of the belt loop going around the pulley gets smaller. When the sheaves are close together, the belt rides higher in the groove, and the radius of the belt loop going around the pulley gets larger.
Thus, the effective radius of both the primary and the secondary pulley is variable. The ratio of the primary pulley radius to the secondary pulley radius determines the ratio of engine rotational speed to the secondary shaft rate of rotation. When the primary clutch radius is smaller than the secondary clutch radius, the secondary shaft will turn at a rate that is slower than the engine speed, resulting in a relatively low vehicle speed. As the ratio of the primary and the secondary clutch radius approaches 1:1, the secondary shaft speed will be approximately equal to the engine or crankshaft speed. As the primary pulley radius becomes greater than the radius of the secondary clutch, an overdrive condition exists in which the secondary shaft is turning at a greater rate than the engine crankshaft. CVT's may use hydraulic pressure, centrifugal force or spring tension to create the force necessary to adjust the pulley halves.
When one pulley increases its radius, the other decreases its radius to keep the belt tight. As the two pulleys change their radii relative to one another, they create an infinite number of gear ratios—from low to high and everything in between. For example, when the pitch radius is small on the driving pulley and large on the driven pulley, then the rotational speed of the driven pulley decreases, resulting in a lower gear. When the pitch radius is large on the driving pulley and small on the driven pulley, then the rotational speed of the driven pulley increases, resulting in a higher gear. Thus, in theory, a CVT has an infinite number of gears through which it can run at any time, at any engine speed or at any vehicle speed.
These variable transmissions are equipped with a speed or revolution per minute (RPM) responsive mechanism associated with the drive pulley and a torque responsive mechanism associated with the driven pulley. Therefore, the drive pulley and the driven pulley continuously vary the shift ratio in relation to the drive speed and the driven torque.
The primary clutch is connected to the power source and in theory has the job of maintaining the engine's RPM at a value where the most power is being produced by the engine. The primary clutch may also control engagement and disengagement of the engine from the load in order to stop and start vehicle movement. In the case of a snowmobile, the secondary or driven clutch is connected to the load through a jackshaft, gears, chain and track, and functions to change the ratio of the two clutches as the load varies. This function is performed by a torque sensing helix or the like, that is typically considered part of the secondary clutch. An example of a secondary clutch having a torque sensing helix is disclosed in U.S. Pat. No. 5,516,333.
As the load to the secondary clutch fluctuates, the torque sensing helix will balance the power being received from the engine and the load by widening or narrowing the distance between the clutch sheaves. Altering the distance between the clutch sheaves changes an effective radius of the clutch around which the drive belt travels. The torque sensing helix is intended to automatically make widening and narrowing adjustments (upshifts and downshifts) almost instantaneously.
The torque sensing helix is essentially a plurality of cam slots formed in a clutch housing. Each cam slot includes cam surfaces that engage associated cam followers that transfer the adjustments made by the torque sensing helix into variations of width between the clutch sheaves. The fixed sheave of the pulley is typically secured to the secondary shaft that transfers a load to and from the vehicle's track or wheels. The clutch housing, including the torque sensing helix is secured to the movable sheave and retains a compression/torque spring against the fixed sheave. The compression/torque spring acts between an end of the housing and the fixed sheave, and is typically adjustable within the housing. The fixed sheave typically has cam followers secured to it that engage the cam surfaces of the torque sensing helix housing. As the torque sensing helix senses a change in load from the secondary shaft, the moveable sheave of the driven clutch will move to either compress or relax the compression spring, causing the cam followers to move up or down the cam surfaces of the helix housing to increase or decrease the radius of the driven clutch.
Thus, when the torque load upon the driven shaft is increased with the increase in the load upon the drive track, as when the vehicle runs uphill, the torque responsive mechanism transmits an axial force matching that increase from the driven pulley to the drive pulley so that a satisfactory shift ratio can always be attained.
The linear, axial movement of the movable sheave with respect to the fixed sheave forms a variable diameter pulley. The linear motion of the sheave along the pivot is controlled by the speed of rotation of the pulley, torque on the transmission belt, and resistance from helical compression springs opposing the linear motion.
One disadvantage with helical compression springs is that they have only a single spring constant, making it difficult to tune for different operating conditions. Another disadvantage with helical compression springs is that they require a relatively large axial dimension to accommodate the spring's minimum depth. Another disadvantage with helical compression springs is that some rotation or twisting occurs during their operation, thereby increasing friction, wear and noise.